I have a relatively new Bottecchia bicycle, about a year and a half year old and underwent about 8,000km.
I almost always use the biggest cog on the front cassette. Lately I noticed that as I apply a lot of torque, i.e. when accelerating from a stop (in high gear), or surely when I stand up, the chain slips -- at least that's what I think that's happening. What I actually experience is a cracking noise, the pedals don't effect the chain for a split second, and then it's back to normal. It is important to say that it doesn't happen to me if I switch to a lower gear on the front crank (but it doesn't matter which gear I'm on at the rear).
I have several ideas as to what may cause the problem. Firstly I thought it may have to do with broken or worn off cog. That is possible because I think I may have over-lubricated the chain last time, and I experienced a sand storm. However I think it may not be the case because it happens regardless of where my pedals are (if I had one broken teeth, for example, I would expect it to happen only when my right pedal is at, say, 7 o'clock or something).
Another possible reason is that maybe I have a problem with the front derailleur -- as I started using the middle gear more often, I noticed that I have problems getting back to the top gear (no problem going down) -- I hear cranking sounds, and often it wouldn't shift until I push the shifter further (even though it doesn't move further, it probably pushes the derailleur a bit, enough to make the jump). It also happened once or twice that under torque the chain slipped to the middle gear.
Third reason I could think of is a worn off chain -- as I said it has served me for probably around 8,000km, and I also noticed there's a noticeable gap between the chain and the wheel (I'll post pictures shortly).
Here are some pictures I took. I hope they're helpful in identifying the cause of it, if not I can take more.
Thank you very much!